1. Field of the Invention
The present invention relates to a temperature compensated type constant-current circuit which is designed to cause the magnitude of a current supplied to a load circuit to be increased or decreased with respect to temperature variation so float a current representing a negative temperature characteristic ls supplied to a load which is likely to be subjected to thermorunaway as a result of current flowing in the circuit being increased with temperature rise as in a power device, for example. More particularly, the present invention pertains to such a constant-current circuit arranged such that the magnitude and temperature charatersitic of the current flowing through the load can readily be set.
2. Description of the Prior Art
Referring to FIG. 1 of the accompanying drawings, there is shown an example of conventional constant-current circuit which is arranged such that current is changed depending on the temperature coefficient of the circuit.
In the illustrated constant-current circuit, a voltage-follower circuit 56 is connected to the output terminal of a temperature characteristic setting circuit 55 coupled to a band-gap reference circuit 54, the output terminal of the voltage-follower circuit 56 being connected to a control resistor R52 which is connected in series with a load 53.
The band-gap reference 54 is a bias circuit which utilizes band-gap potential of a semiconductor and comprises a first PNP type transistor Q51, second PNP type transistor Q52, third NPN type transistor Q53, fourth NPN type transistor Q54, and resistor R51.
Connected in parallel between input terminals 57a and 57a are a series circuit wherein the collector of the first transistor Q51 and that of the third transistor Q53 are connected to each other and the resistor R51 is coupled to the emitter of the third transistor Q53, and another series circuit wherein the collector of the second transistor Q52 is connected to that of the fourth transistor Q54.
The base of the first transistor Q51 and that of the second transistor Q52 are connected to each other, and further the first transistor Q51 has its base connected to its collector. The base of the third transistor Q53 and that of the fourth transistor Q54 are also connected to each other, and the fourth transistor Q54 has its base connected to its collector.
The temperature characteristic setting circuit 55 is a circuit which is arranged to set up the state in which the current derived from the constant-current circuit is changed with temperature, i.e., temperature characteristic in accordance with variations in the current with temperature variations in the band-gap reference circuit 54. This circuit comprises a fifth PNP type transistor Q55, switch SW 51, resistor R54, and diode D51. The fifth transistor Q55 has its emitter connected to the input terminal 57a, and it has also its base coupled to the bases of the first and second transistors Q51 and Q52 of the band-gap reference circuit 54, thus constituting a current mirror circuit. The collector of the fifth transistor Q55 is connected to a movable contact of a change-over switch SW51, and the resistor R54 and diode D51 are connected to a first and a second fixed contact of the switch SW51, respectively. The other terminals of the resistor R54 and diode D51 are coupled to the input terminal 57b.
The voltage-follower circuit 56 is arranged to keep constant the constant voltage across the control resistor R52. More specifically, the circuit 56 comprises an amplifier 52 having an inverting input terminal, a non-inverting input terminal, and an output terminal, wherein the non-inverting input terminal is connected to the output terminal; the output terminal is connected to the control resistor R52; and the non-inverting input terminal is connected to the collector of the fifth transistor Q55 of the temperature characteristic setting circuit 55.
In FIG. 1, V1 is a voltage which occurs at the collector of the fifth transistor Q55 of the temperature characteristic setting circuit 55 and is inputted to the voltage-follower circuit 56; I0 is a current which flows through the load 53 connected across the output terminals 58a and 58b; and I1 is a current which flows in the temperature characteristic setting circuit 55.
Description will first be made of circuit operation which is performed in the ease where the change-over switch SW5 of the temperature characteristic setting circuit 55 is connected to the resistor R51 side.
Current flowing through the second transistor Q52 and that flowing through the fifth transistor Q55 become equal to each other because of the fact that the first and second transistors Q51 and Q52 of the band-gap reference circuit 54 and the fifth transistor Q55 of the temperature characteristic setting circuit 55 constitute a current mirror circuit. Thus, the current I1 flowing through the fifth transistor Q55 of the temperature characteristic setting circuit 55 can be sought from the current flowing in the band-gap reference circuit 54 in accordance with the following equation: EQU I1=(VT/Ra) ln N (1)
where Ra is the resistance value for the resistor R51, and N is the ratio of the emitter area of the third transistor Q53 to that of the fourth transistor Q54. VT is termed thermo-voltage which is a constant given in accordance with the following equation: EQU VT=KT/q (2)
where K is Boltzmann's constant, T is absolute temperature, q is charge mass. The thermo-voltage VT is approximately 26 mV at 27.degree. C.
As will be seen from the above equations (1) and (2), the current I1 includes as a proportional term the thermo-voltage VT having a positive temperature coefficient, and thus it represents positive temperature characteristic. Since the current I1 representing positive temperature characteristic is caused to flow through the resistor R54, the voltage V1 which also represents positive temperature characteristic is applied to the control resistor R52 through the voltage-follower circuit 56. Since the control resistor R52 is connected in series with the load 53, the current I0 flowing through the latter depends on the control resistor R52 and voltage V1 and thus represents positive temperature characteristic.
Circuit operation performed when the change-over switch SW51 of the temperature characteristic setting circuit 55 is connected to the diode D51 side is the same as that when the switch SW51 is connected to the resistor R54 side, in so far as the current I1 represents positive temperature characteristic. Generally, a diode element represents resistance having negative temperature characteristics. If the positive temperature coefficient of the thermo-voltage VT included in the current I1 given by the equations (1) and (2) is greater than the negative temperature coefficient of the resistance of the diode D51, then the voltage V1 turns out to represent negative temperature characteristic. Thus, the current I0 flowing through the load 53 represents negative temperature characteristic.
As will be appreciated from the above discussion, with the conventional constant-current circuit shown in FIG. 1, it is possible to select and set up the temperature characteristic of the current flowing through a load connected thereto, in conformity to the characteristic of the load. However, the temperature characteristic and magnitude of the current I0 is determined by the voltage V1 derived from the temperature characteristic setting circuit 55, and the control resistor R52. Thus, the conventional circuit of FIG. 1 is disadvantageous in that the magnitude and temperature characteristic of the I0 cannot be set up independently so that troublesomeness is experienced in designing the circuit to achieve desired current value and temperature characteristic, and the accuracy with which the current value and temperature characteristic can be set up, is limited.